A new murine aggressive leukemic model

Fact or fiction--identifying the elusive multiple myeloma stem cell

Multiple Myeloma (MM) is a debilitating disease of proliferating and malignant plasma cells that is currently incurable. The ability of monoclonal recurrence of disease suggests it might arise from a stem cell-like population capable of self-renewal. The difficulty to isolate the cancer stem-like cell in MM has introduced confusion toward this hypothesis. However, recent evidence has suggested that MM originates from the B cell lineage with memory-B cell like features, allowing for self-renewal of the progenitor-like status and differentiation to a monoclonal plasma cell population. Furthermore, this tumor-initiating cell uses signaling pathways and microenvironment similar to the hematopoietic stem cell, though hijacking these mechanisms to create and favor a more tumorigenic environment. The bone marrow niche allows for pertinent evasion, either through avoiding immunosurveillance or through direct interaction with the stroma, inducing quiescence and thus drug resistance. Understanding the interaction of the MM stem cell to the microenvironment and the mechanisms utilized by various stem cell-like populations to allow persistence and therapy-resistance can enable for better targeting of this cell population and potential eradication of the disease.

Tumor dormancy: long-term survival in a hostile environment

Tumor dormancy occurs when cancer cells are present but the tumor does not grow. Following treatment, patients may enter complete remission in which persistent cells represent the minimal residual disease (MRD). Experimental models and clinical data suggest that the absolute quantity of this MRD is extremely low. Very few cancer cells can persist for years or decades under these hostile conditions that include continuous exposure to maintenance treatment, autologous anti-tumor immune response, and a nonpermissive microenvironment. Dormant tumor cells may survive despite these destruction factors if they adapt and develop strategies to escape from cell death. Escape may result in a state of equilibrium between MRD and the patient. Equilibrium between the immune response and tumor cells can result in long-term tumor dormancy; however, after variable lengths of time, tumor dormancy ends, and the disease progresses. Experimental models have shown that dormant tumor cells may over-express B7-H1 and B7.1 and inhibit cytotoxic T-cell mediated lysis. This resistance could be therapeutically targeted using drugs like MEK inhibitors that modulate pathways involved in B7-H1 expression. Dormant tumor cells may also develop nonspecific resistance mechanisms to cell death, such as deregulation of JAK/STAT and mTORC2/AKT pathways or autocrine and paracrine production of cytokines. This deregulation leads to cross-resistance between the immune response and cytotoxic drugs, indicating that the long-term selection that occurs in vivo during tumor dormancy may ultimately result in resistant relapse. Long-term selection of cancer cells in vitro using tyrosine kinase inhibitors selects cells that harbor the same resistance mechanisms as dormant tumor cells. Elucidating the mechanisms underlying the equilibrium that allows for the persistence of dormant tumor cells presents a novel strategy for targeted drug treatment in the context of maintenance therapy.

Exploiting mitochondrial dysfunction for effective elimination of imatinib-resistant leukemic cells

Challenges today concern chronic myeloid leukemia (CML) patients resistant to imatinib. There is growing evidence that imatinib-resistant leukemic cells present abnormal glucose metabolism but the impact on mitochondria has been neglected. Our work aimed to better understand and exploit the metabolic alterations of imatinib-resistant leukemic cells. Imatinib-resistant cells presented high glycolysis as compared to sensitive cells. Consistently, expression of key glycolytic enzymes, at least partly mediated by HIF-1α, was modified in imatinib-resistant cells suggesting that imatinib-resistant cells uncouple glycolytic flux from pyruvate oxidation. Interestingly, mitochondria of imatinib-resistant cells exhibited accumulation of TCA cycle intermediates, increased NADH and low oxygen consumption. These mitochondrial alterations due to the partial failure of ETC were further confirmed in leukemic cells isolated from some imatinib-resistant CML patients. As a consequence, mitochondria generated more ROS than those of imatinib-sensitive cells. This, in turn, resulted in increased death of imatinib-resistant leukemic cells following in vitro or in vivo treatment with the pro-oxidants, PEITC and Trisenox, in a syngeneic mouse tumor model. Conversely, inhibition of glycolysis caused derepression of respiration leading to lower cellular ROS. In conclusion, these findings indicate that imatinib-resistant leukemic cells have an unexpected mitochondrial dysfunction that could be exploited for selective therapeutic intervention.

In acute myeloid leukemia, B7-H1 (PD-L1) protection of blasts from cytotoxic T cells is induced by TLR ligands and interferon-gamma and can be reversed using MEK inhibitors

B7-H1 (PD-L1) is a B7-related protein that inhibits T-cell responses. B7-H1 participates in the immunoescape of cancer cells and is also involved in the long-term persistence of leukemic cells in a mouse model of leukemia. B7-H1 can be constitutively expressed by cancer cells, but is also induced by various stimuli. Therefore, we examined the constitutive and inducible expression of B7-H1 and the consequences of this expression in human acute myeloid leukemia (AML). We analyzed B7-H1 expression in a cohort of 79 patients with AML. In addition, we studied blast cells after incubation with interferon-gamma or toll-like receptors (TLR) ligands. Finally, we evaluated functionality of cytotoxic T-cell activity against blast cells. Expression of B7-H1 upon diagnosis was high in 18% of patients. Expression of TLR2, 4 and 9 was detected in one-third of AML samples. Expression of TLR2 and TLR4 ligands or IFN-γ induced by B7-H1 was found to protect AML cells from CTL-mediated lysis. Spontaneous B7-H1 expression was also found to be enhanced upon relapse in some patients. MEK inhibitors, including UO126 and AZD6244, reduced B7-H1 expression and restored CTL-mediated lysis of blast cells. In AML, B7-H1 expression by blasts represents a possible immune escape mechanism. The inducibility of B7-H1 expression by IFN-γ or TLR ligands suggests that various stimuli, either produced during the immune response against leukemia cells or released by infectious microorganisms, could protect leukemic cells from T cells. The efficacy of MEK inhibitors against B7-H1-mediated inhibition of CTLs suggests a possible cancer immunotherapy strategy using targeted drugs.

Micrometastatic disease and metastatic outgrowth: clinical issues and experimental approaches

Metastasis from the primary tumor to distant organs is the principal cause of mortality in patients with cancer. While prognostic factors can predict which patients are likely to have their cancer recur, these are not perfect predictors, and some patient's cancers recur even decades after apparently successful treatment. This phenomenon is referred to as dormancy. Data from experimental studies have revealed two categories of metastatic dormancy: cellular dormancy, with solitary cancer cells in cell-cycle arrest; and micrometastatic dormancy, characterized by a balanced state of proliferation and apoptosis, but with no net increase in size. Development of new models and imaging techniques to track the fate of dormant cancer cells is beginning to shed some light on dormancy. Elucidation of the molecular pathways involved in dormancy will advance clinical understanding and may suggest new avenues for treatment to inhibit the revival of these dormant cells, thereby reducing cancer mortality rates.

BCR-ABL mutants spread resistance to non-mutated cells through a paracrine mechanism

Patients with chronic myeloid leukemia who become resistant to the Abl kinase inhibitor imatinib can be treated with dasatinib. This sequential treatment can lead to BCR-ABL mutations conferring broad resistance to kinase inhibitors. To model the evolution of resistance, we exposed the mouse DA1-3b BCR-ABL(+) leukemic cell line to imatinib for several months, and obtained resistant cells carrying the E255K mutation. We then exposed these cells to dasatinib, and obtained dasatinib-resistant cells with composite E255K+T315I mutations. Subcloning isolated a minor clone also carrying V299L. In co-culture, mutated cells were able to spread resistance to non-mutated cells through overexpression of interleukin 3, activation of MEK/ERK and JAK2/STAT5 pathways, and downregulation of Bim. Even the presence of less than 10% of mutated cells was sufficient to protect non-mutated cells. Blocking JAK2 and MEK1/2 inhibited the protective effect of co-culture. Mutated cells were also sensitive to JAK2 inhibition, but blocking MEK1/2 alone, or in association with kinase inhibitors, had little effect. These data indicate that sequential Abl kinase inhibitor therapy can generate sub-populations of mutated cells, which may coexist with non-mutated cells and protect them through a paracrine mechanism. Targeting JAK2 could eliminate both populations.

Cancer vaccines and tumor dormancy: a long-term struggle between host antitumor immunity and persistent cancer cells?

Tumor dormancy is a phenomenon characterized by the persistence of residual cancer cells for long periods in the host. Evidence has emerged that a balance exists between the immune response and dormant tumor cells. This review presents our current understanding of the immune relationship between host and dormant tumor cells and the mechanism developed by these cells to escape host antitumor immunity. Implications of this immune escape for cancer vaccine strategy are considered.

Cytosine arabinoside induces costimulatory molecule expression in acute myeloid leukemia cells

Chemotherapeutic drugs kill cancer cells mainly by direct cytotoxicity, but they might also induce a stronger host immune response by causing the tumor to produce costimulatory cell surface molecules like CD80. We previously reported that in myeloid leukemic cells, gamma-irradiation induced CD80 expression. In this study, we show that cytosine arabinoside (Ara-C), even at low doses, induced CD80 expression in vitro in mouse DA1-3b leukemic cells, by a mechanism that involved reactive oxygen species. In vivo experiments in the mouse DA1-3b/C3H whole-animal acute myeloid leukemia (AML) model showed that injection of Ara-C induced expression of CD80 and CD86, and decreased expression of B7-H1, indicating that chemotherapy can modify costimulatory molecule expression in vivo, in a way not necessarily observed in vitro. Mouse leukemic cells exposed in vivo to Ara-C were more susceptible to specific cytotoxic lymphocyte (CTL)-mediated killing. Ara-C also induced CD80 or CD86 expression in 14 of 21 primary cultured human AML samples. In humans being treated for AML, induction chemotherapy increased CD86 expression in the leukemic cells. These findings indicate possible synergistic strategies between CTL-based immunotherapy and chemotherapy for treatment. They also suggest an additional mechanism by which chemotherapy can eradicate AML blasts.

Gene transfer of CD154 and IL12 cDNA induces an anti-leukemic immunity in a murine model of acute leukemia

IL12 is an essential cytokine for the generation of T helper 1 response, natural killer (NK) cells and cytotoxic T lymphocyte (CTL) stimulation. CD154 triggers CD40 on antigen-presenting cells, thus inducing antigen presentation to the immune system and production of IL12. As IL12 and CD154 share several pathways mediating immune response, we investigated in an aggressive murine model of acute leukemia the relative antileukemic efficiency of IL12, CD154 and IL12 + CD154 gene transfer. Live leukemic cells transduced by IL12, CD154, and IL12 + CD154 showed reduced leukemogenicity but CD154 protective effect was reduced when 10(6) leukemic cells were injected. Vaccines with lethally irradiated IL12-transduced cells were able to cure mice previously injected with 10(4) leukemic cells and adoptive transfer of IL12-induced antileukemic immunity protected recipient mice. NK cytotoxicity was enhanced in mice vaccinated with leukemic cells transduced by IL12, CD154, and CD154 + IL12. IL12 transduced cells induced IFN-gamma mRNA in CD4(+) and CD8(+) T cells isolated from the spleen of vaccinated animals, however, in vivo depletion experiments showed that IL12 vaccine effect was CD4(+) but not CD8(+) T cell dependent. We conclude that IL12 gene is a more potent candidate than CD154 for gene therapy of acute leukemia.

Gene transfer of GM-CSF, CD80 and CD154 cDNA enhances survival in a murine model of acute leukemia with persistence of a minimal residual disease

Gene transfer of various cytokines and co-stimulatory molecules has been reported to induce a potent antileukemic immunity in murine models, however, the relative efficiency and possible synergistic effects between candidate genes have not been extensively investigated. We analyzed in a murine model of BCR/ABL acute leukemia whether gene transfer of CD154, CD80 or GM-CSF as a single agent or combination of CD154 + GM-CSF, CD80 + CD154 and GM-CSF + CD80 in leukemic cells could enhance survival. We observed that CD154 gene transfer induced a marked inhibition of leukemogenicity, and also that CD154 and combination of GM-CSF and CD80 gene transfer protected mice against subsequent challenge with leukemic cells and had a therapeutic effect for a pre-established leukemia disease. We also found minimal residual leukemic disease by RT-PCR for 6 to 12 months in 0 to 25% of animals injected with transduced leukemic cells and surviving the challenge without evidence of disease, except in the control empty plasmid group where very few mice survived the challenge but all of those were positive by RT-PCR. These findings suggest that leukemic cell vaccination by gene transfer can induce a tumor dormancy phenomenon compatible with long-term survival.